1. Field of the Invention
This invention relates to a method for preparing a high-concentration solids suspension in water.
More particularly, the invention relates to a method which enables a high-concentration aqueous suspension of coal or petroleum coke to be obtained having the characteristic of low sulphur compound emission during combustion.
2. Discussion of the Background
It is well known that during oil or coal combustion sulphur contained in the fuel reacts with oxygen to form sulphur dioxide and sulphur trioxide, a minimum part being retained in the ash, depending on its alkalinity.
An increase in the capacity for absorbing sulphur and its oxides can be obtained by mixing with the fuel or injecting into the combustion chamber alkaline substances such as calcium and/or magnesium oxides, lime, dolomite etc.
The use of sorbents in controlling SO.sub.x emission in boilers fed with fossil fuel is a concept already applied in the past. In the U.S.A. up to the 1960's reductions of between 10 and 40% were obtained in sulphur emission by injecting lime or dolomite into boilers fed with powdered coal. The poor results obtained meant that this technology was considered unsuitable to play an important role in controlling pollutant gas emissions.
Recently however, the issuing by nearly all industrialised countries of more stringent laws regarding emissions has renewed interest in flame desulphurization and has led to various experimental trials both in the U.S.A. and in Europe which have enabled the critical parameters and characteristic quantities concerned in the process to be identified.
In particular, laboratory tests have confirmed the possibility of obtaining good SO.sub.x elimination efficiencies with reasonable Ca/S molar ratios (SO.sub.x reduction of 50-60% on untreated level using a Ca/S ratio of 2), both by flame desulphurization and by adding sorbents to the fuel.
The advantageous path for removing fuel inerts has also been followed in attempts at the cleaner use of coal. Unfortunately, although these methods offers considerable benefits in terms of drastic reduction in ash, sulphur removal reaches a maximum of only 50%, as the common beneficiation methods do not allow the removal of organic sulphur, which on the average represents 50% of the total sulphur present in the coal. Desulphurization methods can also be applied to coal-water mixtures, which have a known composition by weight of 60-75% of suitably ground coal plus 25-40% of water, together with fluidifying additives and if necessary stabilizers (to prevent sedimentation of the solid part) and anticorrosives.
In view of the more restrictive limits on sulphur emission, the application of desulphurization methods to slurries prepared either with coal as such or with beneficiated coal seems to be particularly suitable for satisfying such limits.
However, adding desulphurizers to water/coal or petroleum coke mixtures can negatively affect their rheological characteristics if they are not suitably chosen and metered.